The present invention relates to bypass gas turbine aeroengines, otherwise known as "turbofans", wherein the turbine exhaust gas stream and the bypass air stream are combined with each other before exit from a final propulsion nozzle, the combination of the two streams being facilitated by a "mixer" structure which divides the turbine exhaust stream into a plurality of discrete jets which penetrate and mix with the bypass air stream.
It is known that mixing of the bypass and turbine exhaust streams improves the propulsive efficiency of turbofans by transferring thermal energy from the hot turbine gases to the cooler bypass air. Even small improvements in the efficiency of the mixing process will significantly improve the propulsive efficiency of the turbofan, thereby allowing lower specific fuel consumption. So called "multi-lobed" and "multi-tubed" mixer nozzles, which project portions of the two streams into each other and increase the area of contact between them, have been used to improve the propulsive efficiency by improving the efficiency of the mixing process. During design of such mixer nozzles, the object is to maximise the contribution of the mixer to mixing efficiency whilst minimising mixer weight and the thrust losses inherent in the mixing process itself.